1. Field of the invention
This invention relates to an eyepiece having an aspherical optical member for use in, for example, a telescope such as a binocular, a microscope or the like.
2. Related Background Art
An eyepiece is used, for example, in a telescope such as a binocular, a microscope or the like for enlarging a real image formed by an objective lens for observation. In recent years, an aspherical lens has been introduced in order to reduce the number of lenses constituting an optical system. There have already been proposed eyepieces which employ an aspherical lens in part of constituent lenses. An eyepiece thus having an aspherical lens or an aspherical member therein is hereinafter referred to as an aspherical eyepiece.
Any of conventionally proposed aspherical eyepieces makes part of a spherical lens aspherical to correct pupil aberration mainly caused by the eyepiece. This pupil aberration appears as distortion when a person looks into the eyepiece.
In the prior art, however, since part of a lens constituting an eyepiece is made aspherical, aberration inherent to the eyepiece and the pupil aberration must be corrected simultaneously. To make a sufficient correction for them, there arises a problem that the number of lenses must be considerably increased or the field angle of the eyepiece must be restricted to be narrower. For this reason, even if an aspherical lens is used, for example, in a wide angle eyepiece whose field angle is approximately 50.degree. for use in binoculars, distortion cannot be reduced with a structure having a small number of lenses.